A proving device for proving a flowmeter is a device for subjecting a newly manufactured flowmeter or a flowmeter in use to a characteristic test periodically or at any timing in order for usage at a reliable accuracy regardless of a change in the characteristics attributable to external factors such as temperature and pressure or to internal factors such as abrasion of moving parts. Broadly classified, this characteristic test is carried out by a calibrator that sets a flowmeter to be tested in a fixed tester to perform the test or by a prover type flowmeter tester that sets the flowmeter in a fluid system to perform the test.
Since the prover type can perform the flowmeter characteristic test on line and perform any characteristic test as needed, it is often used in the test of an inferential flowmeter especially susceptible to piping, for example, of a turbine meter. The prover has a moving element such as a piston that moves through a pipe with a constant section in synchronism with a fluid and specifies a fluid ejected by the movement of this moving element through a predetermined interval as a reference volume.
In the flowmeter characteristic test using the prover type, the number of flow pulses sent per unit volume (flow rate coefficient), a so-called K factor is calculated from reading of the flowmeter when a fluid of a reference volume specified by the prover passes therethrough, that is, by detecting the number of flow pulses sent from the flowmeter. Moreover, if necessary, a continuous flow rate characteristic curve is obtained based on the flow rate coefficients in a plurality of flow rates to be measured.
To obtain the flow rate coefficient at a high resolution, the number of flow pulses sent per reference volume needs to be a predetermined number or more and, for example, a specified number of 10,000 pulses or more is given in the case of a large-sized stationary prover having a large reference volume. On the contrary, if the reference volume is reduced, flow pulses of a specified number or more cannot be sent, but the flow rate coefficient can be obtained from a relationship between the reference volume of fluid ejected as a result of movement of the moving element such as the piston and the sent pulses (time) sent from the flowmeter during this time. Accordingly, even in the case of the reduced number of flow pulses, a small-sized prover (small volume prover) is available.
In the above small volume prover (hereinafter, referred to as SVP), a piston prover is known that uses a piston as the moving element. This piston prover has a measurement cylinder with a constant section that is basically connected in series with a flowmeter to be tested and compares the volume of a fluid displaced when the piston moving in the measurement cylinder moves through a certain distance with reading of the flowmeter at that time. The volume of a fluid is actually obtained from an amount of movement of the piston. In the proving, ordinarily, plural times of test results are averaged to calculate the flow rate coefficient (K factor) based on the average value. Consequently, the piston reciprocates by the number of times of the test in the measurement cylinder for each of the flow rates to be measured.
To return the piston to its original position again after completion of measurement by the movement of the piston through a specified interval in the measurement cylinder, the piston is driven against flow of a fluid by an actuator using a hydraulic pressure or a pneumatic pressure via a piston rod, in which for a flow passage allowing a fluid to pass therethrough during this time, there are a case where the measurement cylinder itself is used and a case where a bypass flow passage separately disposed in parallel with the measurement cylinder is used. In the case of causing a fluid to pass through the measurement cylinder, a valve function is provided inside the piston returned by the actuator so that the valve is closed at the time of the measurement and that the valve is opened when the piston is returned. This method is called an internal valve method. Further, in the case of causing a fluid to pass through the bypass flow passage, a bypass valve is disposed in the bypass flow passage so that the valve is closed at the time of the measurement and that the valve is opened at the time of the return. This method is called an external valve method.
Since such an SVP ordinarily has the structure having the piston rod integrally secured to a measuring piston, e.g. when the measuring piston is subjected to a pressure in a rotating direction, there arises a problem that the piston rod works as a resistance to impede the rotation, imposing a load on the measuring piston. In addition, an excessive load on this measuring piston prevents the measuring piston from smoothly moving through the measurement cylinder, making accurate measurement impossible.
On the other hand, for example, a piston prover described in Patent Document 1 is proposed. In this piston prover, a measuring piston and a piston rod are separately constructed, and when returning the measuring piston to a predetermined measurement standby position, the piston rod causes the measuring piston to move from a downstream side to an upstream side. The measuring piston is set at the predetermined measurement standby position, and thereafter only the piston rod is caused to move from the upstream side to the downstream side and is accommodated in a hydraulic cylinder. Thereby, a load on the measuring piston is tried to be reduced.